This technology relates to electrochemical cells and, more particularly, to battery compositions for improving safety and providing overcharge protection at low cost.
Electrochemical storage batteries of all types are susceptible to damage due to overcharging or overdischarging. Overcharging of an electrochemical storage cell in a battery may be defined as charging beyond a cell's design capacity, or at a rate greater than the cell's ability to accept such charge. The damage to the cell which may occur from such overcharging may include degradation of the electrodes, the current collectors, the electrolyte, the binder and the separators between the electrodes. In addition, internal shorting and gas evolution which may result from such overcharging can result in unstable and even dangerous conditions.
Commonly used methods for overcharge protection and cell balancing include system oversizing, external electronic circuits, redox shuttles, and additives.
Underutilization of capacity provides some protection, but at a considerable cost. External electronic circuits are presently used in most commercial lithium-ion battery packs. External circuits monitor the cell and interrupt the charging process when a condition, such as a certain voltage or temperature, is reached. However, these features add weight, cost and complexity to the cell. If one cell in the string is weaker than the others, then the full charge of the whole string is limited by the weakest cell. To control current and monitor the voltage of every cell in a series-connected string requires massive and expensive amounts of wiring and control boards. Furthermore, external circuits can be accidentally disconnected, rendering them ineffective in safeguarding from overcharge.
“Redox shuttles” have been proposed as an approach to solving the problem of overcharging. This approach employs an electrolyte additive which is inactive under normal conditions, but which oxidizes at the positive electrode when the cell potential exceeds the desired voltage, i.e., when the cell is in an overcharge state. The oxidized form of the shuttle additive diffuses through the cell to the negative electrode where it is reduced to its original (unoxidized) state and then the reduced form of the redox shuttle diffuses through the cell back to the positive electrode to continue the redox cycle. The net effect is an internal shunt which prevents damage to the cell by imposing a limit on cell potential. However, the electrolyte additive's ability to carry useful overcharge currents is limited by its diffusion coefficient, which determines the maximum current that can be carried by the redox shuttle (known as the “limiting current density”). Thus, the redox shuttles are limited to applications where the design charging rate is lower than the shuttle's limiting current density. Furthermore, the limiting current density is highly dependent on temperature. Lastly, the limiting current can be increased by increasing the additive's loading in the electrolyte; however, many potentially suitable additives have low solubility in electrolytes.
Some components increase the cell resistance when the cell reaches overcharge voltages, thereby limiting the current for a power-limited charger. However, these methods have the disadvantage that the weakest cell limits the charging of the entire string. Furthermore, they have limited effectiveness to protect a single weak cell in string of a large number of cells.
Systems have been developed to shut down the cell to prevent catastrophic failure from overcharge. For example, current-interrupt devices (CIDs) interrupt the current upon an increase in pressure in the cell, such as may occur from the presence of additives which generate gas when the cell's potential approaches overcharge conditions. However, the cell is permanently incapacitated once the CID is activated.
Other attempts to prevent overcharge of a battery have been proposed by including an electrolyte composition or additive that significantly increases cell impedance of the cell and thereby protects against catastrophic failure of the cell. In some instances, monomers that react upon overcharge are included in the battery. The monomer can, for example, produce gas that trips a pressure-activated disconnect, and/or produce a polymer that clogs the pores in the cell, increasing cell impedance, and/or produce a polymer that forms an electronic shunt. Polymerization of the monomer generally results in substantially increased impedance of the cell, so the cell is protected against catastrophic failure, but is rendered inoperable for future use in the process.
Previous work has looked at using electroactive polymers for overcharge protection. Application of this approach has been limited by the limited number and especially the cost of electroactive polymers having suitable solubility in the casting solvent.